apparently they assumed the velocity of the spaceship is constant, and when viewing from the spaceship, Earth is the one that is moving away at 0.8c, thus the the time on Earth when viewing from the spaceship will be shorter
you dont have to be such a sadist kheir, "laughin your ass off" at the misfortune of others. If you have somethin useful to say then say it, otherwise keep your two cents worth to yourself and have fun in the closet with your printed 2x2cm picture of jezzabelles face with your 2x2cm ......